This invention is pertinent to semiconductor bipolar devices which operate in a high power mode for protracted periods of time relative to the thermal time constant of the semiconductor body itself. In the parlance of the technology, it is desirable to such a device to have a high forward-biased safe operating area. A typical bipolar power transistor is comprised of relatively wide emitter fingers or stripes which have a length long compared with their width. Base contacts are made on one or both sides of the stripe so that base current is supplied to the stripe through the base region underneath the stripe. Because the base region under the emitter must have relatively high resistivity in order to achieve practical values of current gain, there is a substantial voltage drop across the stripe as a result of base current flow. This drop can of course be minimized by reducing the width of the emitter stripe; this approach is a tradeoff with the desired objective of achieving as much active emitter area as possible within a given area of a semiconductor substrate. By reducing the emitter stripe width, relatively more semiconductor substrate area is required for the base contacts which flank the emitter stripe. Thus, to obtain reasonable values of emitter area to semiconductor substrate area, the stripes are usually made of such a width that there is a significant voltage drop in the base underneath the emitter across the width of the stripe. That is, the value of emitter current density is highest at the periphery and falls monotonically to the minimum value in the center of the stripe. If the emitter is too wide, its effective area will be reduced because of the diminished current density at its center. In practical designs, some dimunition of the current density at the center of the stripe is allowed; the stripe width thus selected will often be on the order of or greater than the semiconductor substrate thickness, which thickness is generally minimized in order to reduce the thermal resistance of the substrate. When a device is operated at high power densities for periods of time long compared to its thermal time constant, the non-uniform current density will cause inhomogeneous heating of the device. This inhomogeneous heating of the device can then cause local increases in the value of the current density due to thermal regeneration whereby the power density is even further increased. This result obtains because of the exponential dependence of the collector current density on the local temperature for constant local base emitter voltage. This effect is regenerative because increased current causes increased power and hence further increases in temperature which lead to yet further increases in power. The literature is replete with techniques aimed in reducing this effect. One well-established technique is to introduce series resistance into the base-emitter current path of the device. This technique is most effective when the resistance is introduced in a distributed fashion so that the local current must flow through the local distributed resistance. Clearly, if the local resistance is sufficiently high, the local base-emitter current remains relatively constant. Since the current gain (that is, the collector current divided by the base current) is a relatively slow function of the temperature, the distributed resistance technique is very effective in terms of limiting the local current density. One of the ways in which the distributed resistance can be introduced into the base-emitter path is by placing it in series with the emitter of the device and the emitter metallization means. This has the decided disadvantage that the collector current flow through this resistance will drastically increase the saturation voltage of the device. Another way in which the series resistance can be introduced is by placing it between the edge of the emitter and the base metallization means in contact with the base of the transistor. While this has the advantage of taking the resistance out of the emitter collector current path, this technique too can lead to increased saturation voltages because the base current would prefer to flow into the collector at low values of collector voltage rather than flow through the relatively high distributed resistance in the emitter-base current path.
The distributed resistance techniques recited hereinbefore are most effective when the emitter stripe width is sufficiently narrow that the power density does not vary appreciably across the stripe. That is, they tend to render the average power dissipation uniform down the long dimension of the stripe. It is an object of this invention to provide a device configuration which will minimize the thermal concentration problem across the width of the stripe itself while avoiding some of the increased saturation voltage problems associated with known techniques. It is a further object of this invention to allow the use of relatively wide emitter stripes while avoiding thermal runaway at the edges of the stripes themselves. It is yet another object of this invention to homogenize the power density across the width of an emitter stripe without requiring the use of resistance-introduction techniques which involve the use of fine geometries which inevitably reduce the yield in a semiconductor device. It is yet another object of the invention to provide a technique which can be used in conjunction with known techniques for homogenizing the power distribution down the long dimension of an emitter stripe.
To achieve the above objects, there is described a design which effectively subdivides the narrow dimension of the emitter stripe into a central region flanked by two peripheral regions, which peripheral regions are separated from the central region by an intervening resistive layer. This resistive layer preferably is composed of semiconductor material having the same conductivity type as that of the emitter. By the use of this design, there is a tendency to maintain a more nearly constant emitter-base voltage across the width of the emitter stripe because the lateral voltage drop in the base region beneath the emitter is balanced by a desirably equal but opposite voltage drop across an emitter stripe which is contacted only near its central portion. This technique is distinguishable from distributed resistance techniques utilizing the sheet resistance of an emitter-like region because it is practically impossible to devise emitter and base regions having equal but opposite voltage drops while maintaining desirable values of current gain. That is, compensation of the voltage drops requires that the ratio of the base sheet resistance under the emitter to the emitter sheet resistance be on the order of the current gain, e.g. 100:1, while achieving such typical value of current gain requires a ratio of base sheet resistance under the emitter to emitter sheet resistance on the order of 1000:1 or greater. While exotic geometrical techniques can be used to overcome the inherently undesirable sheet resistance ratio, these techniques are impractical in the sense that they undesirably increase the complexity of the device leading to lower yields, and/or result in a device which has a drastically reduced ratio of effective emitter area to total semiconductor substrate area.